Continental Drift and Plate Tectonics
NCERT Class 11 - Fundamentals of Physical Geography, Chapter 4
Continental Drift and Plate Tectonics
Plate tectonics is the unifying theory of physical geography. It explains why mountains, earthquakes, volcanoes and ocean trenches occur where they do. Its ancestor is Alfred Wegener's continental drift theory of 1912.
Continental drift
- Wegener argued that all land was once a single supercontinent called Pangaea, surrounded by a single ocean called Panthalassa.
- Pangaea first broke into Laurasia in the north and Gondwanaland in the south, separated by the Tethys Sea, and then into the present continents.
- Evidence - the jigsaw fit of the coastlines of South America and Africa; rocks of the same age and type on facing coasts; tillite deposits left by ancient glaciers in India, Africa, South America and Australia; placer deposits of gold in Ghana whose source rock lies in Brazil.
- Fossil evidence - Mesosaurus, a freshwater reptile found only in South Africa and Brazil; the Glossopteris plant found across all the southern continents; and Lystrosaurus in Africa, India and Antarctica.
- The theory was rejected for decades because the forces Wegener proposed - a pole-fleeing force and a tidal force - were far too weak to move continents.
Sea floor spreading
- Ocean floor mapping after the Second World War revealed mid-oceanic ridges, the longest mountain chains on earth, running along the middle of the ocean basins.
- Harry Hess proposed that molten material rises at the ridge, spreads outward on both sides, and is destroyed at deep trenches - the idea of sea floor spreading.
- Evidence - the ocean floor is nowhere older than about 200 million years, far younger than the continents; sediment is thin near the ridge and thicker away from it; and magnetic stripes on either side of the ridge are symmetrical in pattern.
- This supplied the mechanism Wegener lacked, and the modern theory was formulated in the late 1960s.
Plates and their boundaries
- The lithosphere is broken into rigid plates that move over the soft asthenosphere, driven by convection currents in the mantle.
- The seven major plates are the Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic plates; minor plates include the Nazca, Cocos, Arabian, Philippine and Caribbean plates.
| Boundary | Movement | Result and example |
|---|---|---|
| Divergent or constructive | Plates move apart | New crust forms; mid-Atlantic ridge, East African rift valley |
| Convergent, ocean and continent | Ocean plate subducts | Fold mountains and volcanoes; the Andes |
| Convergent, ocean and ocean | One ocean plate subducts | Deep trench and island arc; Japan and the Philippines |
| Convergent, continent and continent | Neither subducts | Crust crumples into high mountains; the Himalayas |
| Transform or conservative | Plates slide past each other | Crust is neither created nor destroyed; the San Andreas fault |
- The Himalayas are the classic continent to continent collision, formed as the Indian plate moved north into the Eurasian plate, closing the Tethys Sea; the range is still rising.
- The Pacific Ring of Fire, where most of the world's earthquakes and volcanoes occur, marks the subduction boundaries surrounding the Pacific plate.
हिंदी संकेत: वेगनर का सिद्धांत इसलिए अस्वीकार हुआ कि उन्होंने जो बल बताए वे बहुत कमजोर थे। सागर नितल प्रसरण ने वही गायब तंत्र दिया, और तभी प्लेट विवर्तनिकी बनी।
Exam me kaise aata hai
Factual questions ask the name of the supercontinent, who proposed continental drift and who proposed sea floor spreading, and the number of major plates. Match-the-following links boundary type with example. Application questions give a landform and ask which boundary produced it.
UPSC / State PSC ke liye
Learn the fossil and rock evidence separately from the mechanism - Wegener's evidence was strong but his mechanism was weak, which is why the theory waited fifty years. Also note that the Himalayas rising and the frequency of Himalayan earthquakes are two faces of the same ongoing collision.
Yahan confuse hote hain
✗ Wegener proposed sea floor spreading | ✓ Wegener proposed continental drift; Harry Hess proposed sea floor spreading
✗ The Himalayas formed at a subduction boundary | ✓ They formed by continent to continent collision, where neither plate subducts
✗ The ocean floor is older than the continents | ✓ It is far younger, nowhere older than about 200 million years
Ek nazar me
- Wegener proposed continental drift in 1912; Pangaea split into Laurasia and Gondwanaland.
- Evidence - jigsaw fit, matching rocks, tillite, placer deposits and fossils such as Glossopteris.
- The theory failed on mechanism until Harry Hess proposed sea floor spreading.
- Ocean floor is nowhere older than about 200 million years; magnetic stripes are symmetrical.
- Seven major plates move on the asthenosphere, driven by mantle convection.
- Divergent boundaries build crust, convergent destroy it, transform neither.
- The Himalayas are a continent to continent collision and are still rising.